Donald Trump decided to go after Ted Cruz about the validity of his citizenship and Presidential eligibility — but Cruz fired back big time.

Republican presidential contenders Ted Cruz and Donald Trump engaged in a heated debate Thursday night over whether Cruz’s Canadian birthplace prevents him from being eligible to hold the office of president.

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“You know, back in September my friend Donald said that he had his lawyers look at this from every which way. And there was no issue there. There was nothing there to this birther issue,” Cruz said. “Now, since September, the Constitution hasn’t changed. But, the poll numbers have. And I recognize that Donald is dismayed that his poll numbers are falling in Iowa.”

“But the facts and the law here are really quite clear,” the Texas senator continued. “Under longstanding U.S. law, the child of a U.S. citizen born abroad is a natural born citizen. If a soldier has a child abroad that child is a natural born citizen. That’s why John McCain, even though he was born in Panama, was eligible to run for president. If an American missionary has a child abroad that child is a natural born citizen. That’s why George Romney, Mitt’s dad, was eligible to run for president even though he was born in Mexico.”

“At the end of the day, the legal issue is quite straightforward, but I would note that the birther theories, that Donald has been relying on, some of the more extreme ones insist that you must not only be born on U.S. soil, but have two parents born on U.S. soil,” Cruz added. “Under that theory, not only would I be disqualified, Marco Rubio would be disqualified, Bobby Jindal would be disqualified, an interestingly enough, Donald J. Trump would be disqualified. Because Donald’s mother was born in Scotland. She was naturalized. Now Donald, on the issue of citizenship Donald, I’m not going to use your mother’s birth against you.”